This invention relates to fluid handling structures such as conduits, vessels, and the like, in which fluent materials are moved or conveyed at high velocities so that high frequency sound waves are generated through the wall of the structure.
In commercial plants, for example, the conveying and processing of material under certain conditions are accomplished by high velocity flow. Under such conditions, particularly when thin walled tubing or structures are used, this high velocity flow of material produces sound waves of high frequency which reverberate from the walled structure through which it is flowing. Such high frequency sound waves produce sharp and irritating noise which is objectionable and which in many cases must be deadened. This is particularly true of sound waves having frequencies in about the 500 Hz range or above.
In U.S. Pat. No. 2,089,492, Lambert recognizes the necessity for reducing sound in air conditioning ducts to remove objectionable noise. The patentee teaches the use of laminated ducts having several layers of sound absorbing material which provide for the reduction of sound propagation as air is conveyed under a pressure head through the ducts. Such sound absorbing materials are good for reducing noise of low frequency; therefore, they are satisfactory for air conveyance under relatively low velocities. The problem in high velocity fluid handling, such as where particulate materials are flowing in a fluid-like manner under a high head of pressure and in many cases in thin walled tubing, sound waves that are produced by the wall of the structure reach frequencies as high or higher than 500 Hz. Such high frequencies produce noise conditions that are above the desirable levels for effective working environment.